It is known, in principle, that blower filter systems are used in situations in which breathing protection is needed. These usually have a blower filter device, in which a blower is arranged for generating an essentially constant volume flow of air. To ensure that only the volume flow being delivered is made available for breathing by the particular person being protected, this person has a respirator device, e.g., in the form of a breathing mask or a respirator hood, placed on his head. The volume flow is delivered by the blower filter device through a tube to the respirator device. The air drawn in is drawn in through a filter in the filter mount in order to achieve a corresponding protective effect. The filters differ especially concerning the substance to be filtered. Filters can be distinguished, in principle, as gas filters and particle filters as well as combined filters, which can filter out gases and particles. The filter capacity decreases in combined filters (in the particle filter part) and in particle filters over the time during which the filter becomes clogged with particles. This reduction in the filter capacity is accompanied by an increased pressure loss, so that an ever-increasing pressure loss is provided through the filter over the duration of use of such a particle filter or of such a combined filter. The blower is then adjusted in prior-art blower devices in order to make it possible to make a constant volume flow available along a characteristic despite the increased pressure loss.
However, the display of the residual capacity is disadvantageous in prior-art blower filter systems. Thus, the residual capacity of a filter is usually calculated by the determined pressure loss of the entire blower filter system. A determination of the pressure loss and hence of the pneumatic resistance of the entire blower filter system is determined in this case as an input variable and compared with the capacity of the blower. The difference of these two parameters forms a basis for the calculation of the residual capacity. The maximum blower value (e.g., 10 mbar) is set as the maximum and a fixed parameter, e.g., approx. 3 mbar, is set as the start value in prior-art calculation systems. However, filters used in different manners have different pneumatic resistances. Filters that have not been used up hitherto consequently have different pneumatic resistances, so that the starting point in the above-described calculation model is, in reality, variable, and depends specifically on the filter. If an especially densely packed filter with a correspondingly high pneumatic resistance is used, a parameter value that is not real but fictitious and is false in this case is used for the start of the value of the capacity for displaying the residual capacity in case of prior-art calculation methods. It may thus happen that the entire system with a densely packed filter already has a basic pressure loss of about 7 mbar while the filter is not used up and correspondingly only a basic overall capacity of 3 mbar to about 10 mbar of the maximum blower capacity is available. However, if the residual capacity is calculated on the basis of a fixed preset value as a starting value of approx. 3 mbar, a corresponding display incorrectly shows that a large portion of the filter has already been used up when using a fresh and unused filter. This leads to reduced acceptance of such a system. In particular, the confidence in the display and hence also in the entire blower filter system as a protective device is reduced. Such an incorrect display possibly also causes the unused filter to be treated as being defective and incorrectly as a reject.